Probably $2-$5 USD a month.
For full paying members I would probably cap. the space at around 500MB, or leave it unlimited.
The only limitation would probably be bandwidth. 1GB costs me about $7, so 1GB would be included (partially funded by the ads), anything over that would be charged at a cut rate or something... But this is the part that is in the air, won't take effect for a while..
At $2 a month, I'd need at least 100 artists participating, and would require 'assistance' in getting them to pay, maybe... we'll see.
Anywho, let me know what you think, or what others are charging, and for what services.
W.